A polymer for photoinduced liquid crystal alignment is used as an agent for liquid crystal alignment of a device using optical characteristics of the liquid crystal. Generally, the polymer for photoinduced liquid crystal means materials which are able to align a liquid crystal by coating a polymer on a substrate and then irradiating ultraviolet rays on its surface to form anisotrophy thereon.
A liquid crystal element using the polymer for liquid crystal alignment is used in various fields including a liquid crystal display, a compensator, optical parts and so on. Thus, though the following description is focused on a display element, it should be understood that the liquid crystal alignment layer is not limitedly used for the display element but able to be applied to various fields described above.
Flat Panel Display (FPD) elements applied to a liquid crystal display device gradually substitute for existing Cathode Ray Tubes (CRT), since the elements are thin and light and capable of scaling-up. Among these FPDs, a Liquid Crystal Display (LCD) leads the FPD market at present because it is advantageous in the facts that it is convenient to carry and consumes low power. In addition, the application of this LCD is broadened to not only calculator and notebook computer but also wall-mounted television and High Definition Television (HDTV).
In order to realize an image with liquid crystal elements, the liquid crystal should be aligned to a predetermined direction on an interface between the liquid crystal and a transparent conductive glass so that the liquid crystal is switched between the transparent conductive glasses owing to an outside electric field. A degree of this liquid crystal alignment is a most important fact for determining the image quality of the ICDs.
Conventionally, there are known three representative methods for aligning a liquid crystal. A first method is a rubbing method shown in FIG. 1, which coats a polymer compound such as polyimide on a substrate and then rubs its surface with a rubbing drum around which a cloth having flock-printed nylon, polyester or rayon fibers is wrapped. A second method is a SiO deposition which deposits SiO on a substrate to an inclined direction. A third method is an alignment method which coats polymers for photoinduced liquid crystal alignment on a substrate and then irradiates a light to a perpendicular or inclined direction in order to cause photoreaction of the coated photopolymer material so that the anisotrophy is formed on its surface.
According to the first method, when rubbing the surface of the polymer compound with the rubbing drum, there are generated minute dusts or electric discharge due to static electricity, which may cause many problems in the liquid crystal panel manufacturing process. According to the second method, the deposition angle to the substrate and the uniformity of a film thickness are hardly maintained, and the process can be enlarged to a large scale. According to the third method, there are problems that a physical coherence between the photoinduced liquid crystal alignment polymer and the liquid crystal is too week and the alignment is weakened due to the heat, which makes the method not be brought into practice.
The liquid crystal alignment using the third method among them is developed to align the liquid crystal by inducing a photoreaction of the photopolymer using light irradiation and thus forming anisotrophy on the coating surface. This alignment method is a non-contact treatment method for the alignment surface and has a feature that the overall process is kept clean since static electricity, dusts or other contaminant particles are not generated. The possibility of the photo-alignment is revealed using an azobenzene compound (K. Ichimura et al. Langmuir, 4, 1214, 1988), and afterward various kinds of polymer compounds such as polymaleimide (H.J.Choi et al. U.S. Patent No. 6,218,501) and polyolefin (R.H.Herr et al. U.S. Patent No. 6,201,087) are developed as a photo-alignment material.
However, in order to put the photo-alignment method to practical use, there are needed improvement of photochemical stability, thermal stability and electrooptical properties and a large amount of ultraviolet rays. Therefore, there is needed the development of a new photo-alignment material to solve such problems.